Tester is in a tight race against GOP Rep. Dennis Rehberg -- Tester’s first reelection bid since winning his initial six-year term in 2006.

The ad’s narration says, in part, "In the past, (Tester has) made a lot of promises, but when he got to D.C., that all changed." Among the examples provided by the ad is that "he’s the largest recipient of lobbyist money." The ad urges voters to "tell" Tester to "stop supporting President Barack Obama’s interests and start supporting Montana’s."

The mention of lobbyist donations caught our attention, since Tester -- a first-term senator with low seniority, no chairmanship and no position in his party’s leadership -- wouldn’t seem to be the most obvious candidate to lead his chamber in lobbyist donations.

We checked with Americans for Prosperity, which pointed us to a chart from the Center for Responsive Politics, a nonpartisan clearinghouse for campaign finance information.

The chart, labeled "Lobbyists: Top Recipients," shows Tester leading the pack with $348,031.

This seemed pretty persuasive, until we spot-checked some of the most influential Democrats and Republicans in the Senate to see whether Tester really outraised them. It turns out he didn’t.

To clarify matters, we checked with the Center for Responsive Politics. It turns out that the top-20 list that Tester headed only includes donations by registered lobbyists made to candidates’ campaign committees during the 2011-2012 period -- the period in which Tester, and everyone else on that list, is up for reelection.

Since donors typically give to campaign committees most heavily during the two-year cycle when a candidate is actually up for reelection, Tester was, in effect, only No. 1 in the chamber when compared to candidates currently running for reelection -- not against the entire Senate. None of the five senators we listed above who have higher totals are up for reelection this year. Consequently, none of them has made it onto the top-20 list for 2011-2012.

If you equalize the playing field by looking at a full six-year senatorial term, Tester is no longer No. 1 in lobbyist donations, though because of how the Senate staggers the reelection years of its six-year terms, there’s not a complete ranking of all 100 senators. (In a CRP's all-inclusive list of senators who have served since 1989, Tester barely cracks the top 50 in lobbyist donations. The upper levels of this list include many one-time senators who have been out of office for years, such as Al Gore, Hillary Clinton and Barack Obama.)

Still, it’s somewhat misleading for the ad to say, without further qualification, that Tester is "the largest recipient of lobbyist money" among senators.

Our ruling

Americans for Prosperity claimed that Tester is "the largest recipient of lobbyist money" in the Senate, and it provided as backup a chart from a credible, independent organization showing Tester topping the list. However, the ad doesn’t clarify an important quirk in how CRP puts together its rankings: The ranking is based only on donations made this election cycle. That makes the ad’s phrasing somewhat misleading, because donations tend to spike during a senator's re-election cycle, and looking just one cycle essentially excludes other senators who have accepted more money than Tester if the comparison had been made on an apples-to-apples basis. We rate this claim Half True.